I do not know if this is relevant, but I recently purchased a radeon 6450 and
had similar issues - corruption when KMS is started and lockups when X starts.
I did not get these issues in fglrx or the windows drivers.

I then happened to notice that the memory clock settings for the 'boot' power
profile were incorrect, and were significantly higher than the card should
support. The high & low power states appeared to be correct. Changing the boot
values using the Radeon Bios editor (http://www.techpowerup.com/rbe/) fixed
these problems for me.